A variety of kinds of radiation image capturing apparatus such as a so-called direct-type radiation image capturing apparatus which generates an electric charge according to the dose of the emitted radiation such as X-ray and converts the electric charge thus generated into an electric signal and a so-called indirect-type radiation image capturing apparatus which converts emitted radiation into an electromagnetic wave having other wavelength such as a visible light by way of a scintillator or the like and then generate an electric charge by a photoelectric conversion element such as a photodiode according to the energy of the converted and emitted electromagnetic wave to convert the electric charge into an electric signal has been developed. Note that in the present invention, a detecting element of the direct-type radiation image capturing apparatus and a photoelectric conversion element of the indirect-type radiation image capturing apparatus will be collectively called as a radiation detection element.
This type of radiation image capturing apparatus has been known as a flat panel detector (FPD) and has been conventionally formed in an integrated manner with a supporting board (or a Bucky device) (for example, refer to the Patent Document 1). However, in recent years, a portable-type radiation image capturing apparatus including a housing in which the radiation detection element and the like are stored has been developed and practically used (for example, refer to the Patent Documents 2 and 3).
In such a radiation image capturing apparatus, as shown in later-described FIG. 3 or FIG. 7, for example, radiation detection elements are aligned two-dimensionally (in a matrix manner) on a detecting part P and a switching element having a thin film transistor (hereinafter referred to as TFT) 8 is provided to each of the radiation detection elements 7. Then, in many cases, before capturing a radiation image, that is, before the radiation image capturing apparatus is irradiated with radiation emitted from a radiation generating device, on/off of the TFT 8 is appropriately controlled while reset process to release unnecessary electric charges left in each radiation detection element 7 is carried out.
Then, after the reset process of each radiation detection element 7 is completed, an off-state voltage is applied to the TFT 8 from a gate driver 15b of a scanning drive unit 15 via each scanning line 6 to cause all the TFTs 8 to be in an off-state. Then, when the radiation image capturing apparatus is irradiated with radiation emitted from the radiation generating device, electric charges according to the dose of the radiation are generated in each of the radiation detection elements 7 and accumulated in each of the radiation detection elements 7.
Then, in many cases, the radiation image capturing apparatus is configured so that after the irradiation of the radiation to the radiation image capturing apparatus (that is, after the radiation image capturing is carried out), as shown in FIG. 40, while each of the lines L1 to Lx of scanning lines 5 for applying an on-state voltage from the gate driver 15b of the scanning drive unit 15 for readout of signals are sequentially switched, the electric charges accumulated in each of the radiation detection elements 7 are read out and charge/voltage converted or the like by a readout circuit 17 so that the electric charges are read out as image data.
However, in such a configuration, it is necessary that an interface is precisely constructed between the radiation image capturing apparatus and the radiation generating device for irradiating the radiation image capturing apparatus with the radiation and when the irradiation is performed, the radiation image capturing apparatus must be in a condition where the radiation image capturing apparatus can accumulate the electric charges in the radiation detection elements 7. However, it is not necessarily easy to construct the interface between the devices. Moreover, if the irradiation is performed while the radiation image capturing apparatus is carrying out reset process of each of the radiation detection elements 7, the electric charges generated by the irradiation of the radiation are flown out from each of the radiation detection elements 7, thus causing such problems that the conversion ratio of the emitted radiation to the electric charges, that is, conversion ratio of the emitted radiation into the image data is lowered.
Therefore, various techniques for detecting irradiation with radiation by the radiation image capturing apparatus itself have been developed in recent years. As a part of such techniques, detection of irradiation with radiation by the radiation image capturing apparatus itself has been considered by use of, for example, techniques described in the Patent Document 4 or 5.
According to the Patent Documents 4 and 5, descriptions are made in which a radiation image capturing apparatus or a method for reading out image data by repeatedly carrying out the readout process of the image data from the radiation detection elements 7 while sequentially switching each of the lines L1 to Lx of the scanning drive line 5 for applying the on-state voltage from the gate driver 15b of the scanning drive unit 15 while the radiation image capturing apparatus is irradiated with radiation.
In this case, as shown in FIG. 41, if a period during which the on-state voltage is sequentially applied to each of the lines L1 to LX of the scanning drive line 5 and each image data is read out from each of the radiation detection elements 7 which are targets for reading out the image data among all the radiation detection elements 7 aligned on the detecting part P is set to be 1 frame, the electric charges generated in the radiation detection elements 7 by the irradiation with the radiation is read out in a divided manner in each frame.
Therefore, image data read out for each frame from a frame in which irradiation with the radiation is started to a frame following a frame in which irradiation with the radiation has completed are added for each of the radiation detection elements 7 and thus the image data for each of the radiation detection elements 7 are reconstructed.
However, according to a study conducted by the inventors of the present invention, it became clear that following problems would arise if readout process of image data is continuously carried out for each frame after detecting irradiation with radiation, as in the invention described in the Patent Documents 4 and 5.
That is, in this case, as shown in FIG. 42, in a case where readout process of image data is carried out for each frame while on-state voltage is sequentially applied from the top scanning line 5 of the figure to each of the scanning lines 5 by the gate driver 15b, and, for example, irradiation is performed onto the shaded portion of FIG. 43, which is denoted by ΔT, while the on-state voltage is applied to the scanning line 5 in the shaded portion and the irradiation is completed. Here, FIG. 43 does not indicate that the irradiation is performed only to the shaded portion ΔT but also onto the entire area of the detecting part P.
Then, if the readout process of the image data is continued after subsequently to the above to carry out the readout process of the image data, and, as mentioned above, image data of each frame including this frame for two or three times are added to reconstruct image data for each radiation detection element 7, there appears unevenness in thickness of shade in the radiation image generated on the basis of thus reconstructed image data, as shown in FIGS. 44A and 44B.
That is, for example, in a radiation image generated based on each image data d which was reconstructed by irradiating the entire area of the detecting part P of the radiation image capturing apparatus uniformly with the same dose of radiation, when each of the reconstructed image data d is seen along a stretching direction of the signal line 6 (direction indicated by a vertical arrow in FIG. 44A), as shown in FIG. 44B, the image data d in an image area 5T corresponding to the scanning line 5 on which the on-state voltage was sequentially applied while radiation was irradiated (that is, the shaded portion ΔT of FIG. 43) has a larger value than the image data d of an image area A above the image area δT or an image area B below the image area δT.
Therefore, the portion of the image area δT of the radiation image becomes slightly thicker (that is, darker) compared to the image area A or image area B. Thus, it is known that there occurs a problem that despite uniform irradiation with radiation to the radiation image capturing apparatus, there appears unevenness in thickness.
This is not limited to a case where the entire area of the detecting part P of the radiation image capturing apparatus is uniformly irradiated with the same dose of radiation, but even in a case where irradiation is performed to the radiation image capturing apparatus through a subject, unevenness in thickness appears in the generated radiation image in a similar manner.
The reason why the image data d of image area δT becomes larger than the image data d in the image areas A and B is assumed as follows.
That is, as shown in FIG. 45, in a case where an image data di is read out from a radiation detection element 7i when the on-state voltage is applied to a line Li of the scanning line 5, a small amount of electric charge q gradually leaks out from the radiation detection element 7, which is connected to another line L of the scanning line 5 to which an off-state voltage is applied simultaneously, via a TFT 8. Therefore, the image data di read out as the image data of the radiation detection element 7i is actually an image data corresponding to a sum of the electric charge Q read out from the radiation detection element 7i and the electric charge q leaked out from the other radiation detection element 7 via the TFT 8.
Moreover, in a case where readout process is carried out while irradiation is performed to the radiation image capturing apparatus 1, each of the TFT 8 is irradiated with the same radiation as the radiation image capturing apparatus 1, or the radiation thus emitted is converted into an electromagnetic wave by a scintillator and the electromagnetic wave is incident onto each of the TFT 8 to increase the amount of the electric charge q leaked out from the radiation detection element 7 via each of the TFT 8.
Therefore, in this case, the image data di readout as the image data of the radiation detection element 7i becomes larger for the amount of each electric charge q leaked out from the other radiation detection element 7 connected to the same signal line 6. It is therefore assumed that the image data d of the image area δT becomes larger than the image data d in the image areas A and B.
However, if unevenness in thickness appears in the radiation image generated as above, it becomes difficult to see the radiation image. Moreover, for example, in a case where the radiation image is used for medical diagnosis, if the uneven portion and lesion location overlap on the radiation image, there is a possibility that the lesion location is missed or mistaken. In addition, it is not necessarily easy to correct the image data d of the image area δT which became larger than the image data d in the image areas A and B as shown in FIG. 44B.
Therefore, it is conceivable to configure a device which carries out readout process before start of the irradiation with radiation to the radiation image capturing apparatus by applying the invention described in the Patent Documents 4 and 5 and, instead of continuously carrying out the readout process while the radiation image capturing apparatus is irradiated with radiation as in the invention of the Patent Documents 4 and 5, stops carrying out the readout process of the image data d at the moment when irradiation with radiation is initiated.
If the device is configured as above, the image data d is read out from each of the radiation detection elements 7 connected to the scanning line 5 to which the on-state voltage is applied from the gate driver 15b of the scanning drive unit 15, the image data d having a significantly larger value than the image data d read out from each of the radiation detection elements 7 connected to the scanning line 5 to which the on-state voltage has been previously applied.
Therefore, it becomes possible to configure a device by use of this phenomenon that, for example, carries out the readout process of the image data d before irradiation with radiation to the radiation image capturing apparatus is started and in a case where the image data d thus read out suddenly increases and exceeds a threshold value, detects that irradiation with radiation started. Then, if initiation of irradiation with radiation is detected, the readout process of the image data d is stopped and the electric charges generated in each of the radiation detection elements 7 by the irradiation with radiation are caused to be accumulated in each of the radiation detection elements 7.
Moreover, though not shown, if an electric current detection unit is provided to a bias line 9 connecting each of the radiation detection elements 7 or to a wire connection 10 which bands them together (refer to later described FIG. 7 and the like) (for example, refer to the Patent Document 6), or if an electric current detection unit is provided to each of the scanning lines 5 or a wire 15c connecting the scanning drive unit 15 and the gate driver 15b, current value detected by the electric current detection unit suddenly increases when the radiation image capturing apparatus is irradiated with radiation.
Therefore, by use of the above, it becomes possible to configure a device which monitors the value of the electric current detected by the electric current detection unit and if the current value suddenly increases and, for example, exceeds a previously set threshold value, detects that the radiation image capturing apparatus has been irradiated with the radiation at that moment. Then, in this case also, when initiation of irradiation with radiation is detected, the readout process of the image data d (or in this case, reset process of each of the radiation detection elements 7 for causing each radiation detection element 7 to release the electric charges remaining in each of the radiation detection elements 7 may be applicable) it stopped and the electric charges generated in each of the radiation detection elements 7 by the irradiation with radiation are caused to be accumulated in each of the radiation detection elements 7.
Then, by configuring as mentioned above, it becomes possible to detect irradiation with radiation by the radiation image capturing apparatus itself in a case where it is impossible to construct an interface between the radiation image capturing apparatus and the radiation generating device.
Meanwhile, an image data readout after irradiation with radiation by the radiation image capturing apparatus as described above (for the purpose of distinction between the image data read out before photographing by the radiation image capturing apparatus described above, this data shall be referred to as the “image data D” hereinafter) includes the image data D attributable to the electric charge generated in each of the radiation detection elements 7 by the irradiation with radiation to the radiation image capturing apparatus as described above (hereinafter, this image data D shall be referred to as “true image data D*”, meaning that this image data is attributable to the true electric charge generated by the irradiation with radiation).
However, other than the above, due to the thermal excitation or the like of each of the radiation detection elements 7 itself, a so-called dark electric charge is always generated in each of the radiation detection elements 7 and when the image data D is read out from each of the radiation detection elements 7, in addition to the true image data D*, an off-set data O which is an off-setting portion attributable to the dark electric charge is also read out. In other words, the image data D is expressed as a sum of the true image data D* and the off-set data O attributable to the dark electric charge, as in the following equation (1):D=D*+O  (1)
Then, because the data which should be acquired as image data is the true image data D* attributable to the true electric charge generated by the irradiation with radiation, the radiation image capturing apparatus is configured to obtain the off-set data O attributable to the dark electric charge under the same condition as photographing a radiation image but the radiation image capturing apparatus is not irradiated with radiation before or after photographing of the radiation image to calculate the true image data D* according to the following equation (2) obtained by modifying the above mentioned equation (1):D*=D−O  (2)